The invention relates to an X-ray diagnostic installation, comprising an image intensifier television chain, comprising a subtraction device, which exhibits at least one image memory and a difference stage for providing subtraction images through difference formation from the stored video signals and chronologically displaced video signals, and comprising a monitor. Subtraction images are employed in angiography in order to render clearly visible blood vessels which in the normal X-ray image can be only poorly recognized and on which bone structures are superimposed.
In German Pat. No. 30 18 129 and in the corresponding U.S. Pat. No. 4,398,213 issued Aug. 9, 1983, an X-ray diagnostic installation for providing subtraction images is described in which, in a memory, a blank image averaged over several scannings, i.e. an image without contrast medium filling, is stored. Subsequently, a contrast medium is injected. The filling images are now either directly supplied to a difference stage (fluoroscopy subtraction) or stored in an additional image memory, averaged over several scannings. In the difference stage the subtraction of the blank image from the filling image proceeds, so that, at a series connected monitor subsequently only the filled vessels which are of interest can be seen.
The storage of the images proceeds in digital image memories. The subtraction and the further processing are likewise digitally conducted. To this end, the video signals are digitized in an analog-to-digital converter (A/D converter) and, following completed processing, are reconverted to analog video signals in a digital-to-analog converter (D/A converter) and displayed on a monitor. The conventional A/D converters, given a clock pulse frequency corresponding to the video image rate may exhibit an amplitude depth of eight bits. Indeed, A/D converters with a higher amplitude depth can also be employed. However, the latter presently operate with a lower clock pulse frequency so that fluoroscopy records can be provided only in a time-delayed fashion. However, losses of definition due to movement can result under these circumstances.
The vessels illustrated in the digital subtraction technique have, in the spatial frequency spectrum, a predominant spectral component with high spatial frequencies. Through the modulation transfer function of the X-ray image intensifier chain, there result, for these spectral components, relatively low amplitude values in comparison with the remaining image contents which are provided by the contrast of greater surface components. Therefore, in the case of a conventional X-ray video signal, relative to the vessel representation, a relatively poor utilization of the amplitude range of the analog-to-digital converter is attained. The resolution is thereby strongly restricted and one obtains a low signal-to-noise ratio.